Derivative Instruments Derivative Instruments - Offsetting Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative Asset [Abstract]
Total derivative assets, gross amounts recognized	$ 135.4	$ 157.5
Total derivative assets, gross amounts offset in the balance sheet	0	0
Total derivative assets, net amounts presented in the balance sheet	135.4	157.5
Total derivative assets, gross amounts not offset in the balance sheet, financial instruments	(73.4)	(82.6)
Total derivative assets, net amount	62.0	74.9
Offsetting Derivative Liabilities [Abstract]
Total derivative liabilities, gross amounts recognized	(77.1)	(85.6)
Total derivative liabilities, gross amounts offset in the balance sheet	17.1	13.0
Total derivative liabilities, net amounts presented in the balance sheet	(77.1)	(85.6)
Total derivative liabilities, gross amounts not offset int he balance sheet, financial instruments	73.4	82.6
Total derivative liabilities, gross amounts not offset in the balance sheet, cash collateral pledged	3.7	3.0
Total derivative liability, net amount	$ 0	$ 0